Every F[0-9]+ key is two things on the Mac. It's a function key, with its definition set by a carefully negotiated contract between the OS and the current application, or it's a non-changeable controller with a pretty picture (volume, screen brightness, etc.)
You do one by pressing the key, and the other by pressing it with the Fn modifier key. Which one is default depends on a pref setting in Keyboard & Mouse > Keyboard Shortcuts.
Personally, I set it to use the F[0-9]+ keys as function keys, and use the Fn modifier to access the volume etc.
Sorry about that, I checked the box in the settings that reversed function in relation to F11 being pressed. My primary point, that it doesn't make Firefox go full screen, still stands.